CXO Project Science: X-ray opacity probabilities from HRC-I +Z vertex

Figures show probability distributions of 60-keV absorption depth, as viewed from the HRC-I +Z vertex (worst-case location), for 3 locations of the detector. Solid line denotes HRC-I on the HRMA axis; dashed, ACIS-I; and dotted, ACIS-S. Note that x-ray absorption is roughly inversely proportional to the cube of the photon energy; thus, an absorption depth 0.04 at 60 keV corresponds to about 1 at 20 keV.

Of the 2 HRC Next-in-Line (NiL) locations, the more benign x-ray environment is with ACIS-S on the HRMA axis. However, low-opacity lines of sight are much more probable while in NiL position than during HRC-I pointed observations. Furthermore, this is true whether or not the tantalum shielding is on the focus structure.

HRC-I +Z vertex, with full focus-structure shielding

With full focus-structure shielding and the HRC-I on the HRMA axis, the HRC-I worst-case vertex is shielded in essentially all directions up to about 50 keV. However, even in the more benign HRC Next-in-Line (NiL) position, about 2.5% of the sky (0.3 sr) is transparent down to about 20 keV.

HRC-I +Z vertex, with no focus-structure shielding

With no focus-structure shielding and the HRC-I on the HRMA axis, only about 0.25% of the sky (0.03 sr) is transparent down to about 20 keV at the HRC-I worst-case vertex. However, even in the more benign HRC Next-in-Line (NiL) position, about 3.5% of the sky (0.4 sr) is transparent down to about 20 keV.

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